Researchers collected data from the British National Child Development Study (NCDS).
This longitudinal study followed a group of people born in the same week in 1958, starting at age 7.
The researchers examined participant surveys collected at age 42, 45 and 50 that included questions about their psychological health and the number of friends and relatives they met up with once a month or more.

Forty percent of men and roughly a third of women said they had more than six friends whom they saw regularly.
Those people were significantly happier than their peers.
Moreover, maintaining relationships is key: “Having fewer than ﬁve friends at age 45 predicted signiﬁcantly poorer psychological well-being at age 50,” the researchers write.

Source: British National Child Development Study (NCDS), a longitudinal study, UK, 2010